Sedimentary and Diagenetic Features and Their Impacts on Microbial Carbonate Reservoirs in the Fourth Member of the Middle Triassic Leikoupo Formation, Western Sichuan Basin, China

In recent years, the discovery of two gas fields in the fourth member of the Leikoupo Formation in the Western Sichuan Basin of SW China confirmed the exploration potential of microbial carbonates. The aim of the present study is to clarify the formation mechanism of the microbial reservoirs in the Leikoupo Formation. For this purpose, lithofacies, depositional environments, and diagenesis analyses were performed in samples collected from cores of 12 wells. The climate of study area was arid during Anisian time, and the water body was restricted. In such a climate, an evaporitic environment was developed, where ten types of lithofacies, dominated by microbial carbonates and gypsum rocks, were recognized. Thrombolites and stromatolites are the main high-quality reservoirs rock types in the fourth member of the Leikoupo Formation in the Western Sichuan Basin of SW China, which developed as microbial mounds, with reservoir space of microbial inter-clot pores, intra-clot pores, fenestral pores, inter-crystalline pores, and cracks. The microbial inter-clot pores are the main reservoir space, formed by trapping and binding of marls by benthic microbial communities. These pores were partially filled with evaporites because of the arid climate, which were subsequently dissolved (mainly gypsum) in the syn-depositional period, thus greatly improving the quality of reservoirs. Although some pores were occluded by multi-stage cements during the burial stage, major pores were well preserved own to the early dolomitization, rapid burial of the Leikoupo Formation, and early charging of hydrocarbon. The early dolomitization enhanced the anti-compaction ability of microbial carbonates during the burial stage. Rapid burial of the Leikoupo succession slowed down early cementation, and it also accelerated the maturation and expulsion process source rock to promote early charging of hydrocarbon in pores, which created a closed system, inhibiting strong burial cementation.

. The location of the study area in Middle Triassic. (b). The location of the study area in modern times. (c). The present tectonic units and representative wells of the study area (modified from [48]).
The subduction of the Yangtze block caused the ancient Tethys Ocean to be gradually closed, and the seawater gradually withdrew from the Western Sichuan Basin in the NE-SW direction. The climate was arid at this time. In such a climate, an evaporitic environment was developed, and the water body was restricted [50,51]. The total thickness of Lei-4 ranges from 144 to 2047 ft (44 to 624 m), and it can be divided into three sub-members, based on the different  Under the control of the tectonic-sedimentary evolution, the Feixianguan Formation (the Lower Triassic), the Jianglingjiang Formation (the Lower Triaasic), the Leikoupo Formation (the Middle Triassic), the Ma'antang Formation (the Upper Triassic), and the Xujiahe Formation (the Upper Triassic) developed during the Triassic in the Western Sichuan Basin. The Leikoupo Formation was divided into four members, that is, from bottom to top, Lei-1 (T 2 l 1 ), Lei-2 (T 2 l 2 ), Lei-3 (T 2 l 3 ), and Lei-4 (T 2 l 4 ) [16,34,35] (Figure 2). Lei-4 was eroded to various degrees in the Western Sichuan Basin because of the Indosinian Movement, and the remaining thickness gradually decreased from west to east [49]. Lei-4 is mostly unconformably overlain by the Upper Triassic in the east and south of the Sichuan Basin, while in the west of the Sichuan Basin, especially in the Yazihe and Huanglianqiao areas, it is conformably overlain by the Tianjingshan Formation [34].

Samples and Analytical Method
A total of 300 core samples were obtained from 12 wells in the study area ( Figure 1). In one-third of the region, 300 thin sections were stained by Alizarin Red S to distinguish calcite and dolomite. Each thin section had a thickness of 0.03 mm and was impregnated with blue dye to reveal mega-pores.
Measurements on the microporosity were performed in 25 representative samples (the The subduction of the Yangtze block caused the ancient Tethys Ocean to be gradually closed, and the seawater gradually withdrew from the Western Sichuan Basin in the NE-SW direction. The climate was arid at this time. In such a climate, an evaporitic environment was developed, and the water body was restricted [50,51]. The total thickness of Lei-4 ranges from 144 to 2047 ft (44 to 624 m), and it can be divided into three sub-members, based on the different lithological associations ( Figure 2). The lower Lei-4 (T 2 l 4 1 ) has a thickness of 72-699 ft (22-213 m) and is dominated by thick anhydrite, anhydrite-dolomite, and dolomicrite, which are completely preserved in the Western Sichuan Basin. The middle Lei-4 (T 2 l 4 2 ) is similar to the lower in terms of thickness (72-

Samples and Analytical Method
A total of 300 core samples were obtained from 12 wells in the study area ( Figure 1). In one-third of the region, 300 thin sections were stained by Alizarin Red S to distinguish calcite and dolomite. Each thin section had a thickness of 0.03 mm and was impregnated with blue dye to reveal mega-pores.
Measurements on the microporosity were performed in 25 representative samples (the length, width, and height are all less than 3 mm) by a scanning electron microscope (FEI, USA), equipped with Energy Disperse Spectroscopy, (EDS) at the Key Laboratory of Orogenic Belts and Crustal Evolution, Peking University.
In order to quantificationally clarify the main pore type of the microbial reservoirs using the Image-J software, the percentage of each pore type area to the total pore area was counted through the following steps: (1) Representative images of thin casting sections with an area of 1.6 × 10 8 µm 2 (magnification of 50 times using a polarizing microscope) are taken; (2) the image analysis technology of Image-J is used to identify the pores and the total area of pores in the area, and the areal porosity is counted by S pore /S total × 100%; (3) different types of pores are artificially recognized, and the percentage of each pore type to the total pore area is counted by S single pore /S total pore × 100%.
For CL analysis, 40 selected samples were polished into thin Sections (0.06 mm thick) to identify the periods of cementation. Polarized light images were obtained using Leica DM2500P, and the CL images were acquired from CL8200 MK5 in a high vacuum field at the Petroleum Geology Research and Laboratory Center, Research Institute of Petroleum Exploration and Development, Petro China.
Powder samples of microbial carbonates were obtained for trace elements and rare earth elements (REE) at the Key Laboratory of Orogenic Belts and Crustal Evolution, Peking University. Of the powder samples, 250 mg were digested with 5 mL of HNO 3 for 1 h and then dried. Then, 10 mL of HNO 3 (1.42 g/mL) was added to the dried sample and heated at 130 • C for 2 h. Finally, an inductively coupled plasma-mass spectrometer (ICP-MS) was used to determine the contents of trace elements, including rare earth elements. ICP uses a high-frequency RF signal, applied to the inductor coil, to form a high-temperature plasma inside the coil. The high-temperature plasma ionizes all elements in the sample to form a monovalent positive ion and then transmits the monovalent positive ion to the mass spectrometer. The mass spectrometer can screen out ions with different ratios of mass to nuclear (m/z) and detect the intensity of each ion to analyze the intensity of each element. The values of rare earth elements are normalized to the Post-Archen Australian Shale (PAAS), published by Ali in 2012. The Eu and Ce anomaly values were calculated as follows: δEu = Eu*/(0.5Sm* + 0.5Gd*), and δCe = Ce*/(0.5La* + 0.5Pr*) [52].
Powder samples of microbial carbonates were selected for C and O isotopic measurements at the Key Laboratory of Orogenic Belts and Crustal Evolution, Peking University. The instrument was the IsoPrime 100 mass spectrometer, and the error of the results was within ±0.1% . The test steps were as follows: (1) Every single sample is placed into a drying baker at 105 • C for 2 h to remove adsorbed water; (2) 20 mg of pure sample is weighed on the balance and put into the bottom of the reactor; (3) 4 mL anhydrous phosphoric acid is injected into the branch of the reactor, and the anhydrous phosphoric acid is heated at 60-70 • C for 2 h, until no more bubbles are produced; (4) the anhydrous

Thrombolites
Thrombolites are comprehensive products of the capture from cyanobacteria or other archaea, self-calcification of microorganisms, precipitation of carbonates and diagenesis [53]. In this study, thrombolites (facies A) mainly developed in T 2 l 4 3 .
Core observations showed that the microbial adhesion structure (Figure 3a) developed in thrombolites, and a large number of pinholes and irregular dissolved pores, could be found (Figure 3b). Thin section observation under a microscope showed that two types of clots occurred in the thrombolites: an irregular-shaped clot, composed of uniform dolomicrite, without any obvious internal sedimentary structure; and an irregular-shaped clot bond of peloids ( Figure 4a). Three types of benthic foraminifera were found in thrombolites: miliolinid foraminifera (Figure 4b), agglutinated textulariid foraminifera, with uniserially arranged chambers, and agglutinated textulariid foraminifera, with biserially arranged chambers. The typical Girvanella, a common cyanobacterium, could be found in the thrombolites (Figure 4c).
Thrombolites are comprehensive products of the capture from cyanobacteria or other archaea, self-calcification of microorganisms, precipitation of carbonates and diagenesis [53]. In this study, thrombolites (facies A) mainly developed in T2l4 3 .
Core observations showed that the microbial adhesion structure (Figure 3a) developed in thrombolites, and a large number of pinholes and irregular dissolved pores, could be found ( Figure 3b). Thin section observation under a microscope showed that two types of clots occurred in the thrombolites: an irregular-shaped clot, composed of uniform dolomicrite, without any obvious internal sedimentary structure; and an irregular-shaped clot bond of peloids ( Figure 4a). Three types of benthic foraminifera were found in thrombolites: miliolinid foraminifera (Figure 4b), agglutinated textulariid foraminifera, with uniserially arranged chambers, and agglutinated textulariid foraminifera, with biserially arranged chambers. The typical Girvanella, a common cyanobacterium, could be found in the thrombolites (Figure 4c).
Another type of thrombolite is anhydritic thrombolites (facies B), which was developed mainly in T2l4 1 and T2l4 2 . The characteristics of the clots in the anhydritic thrombolites were similar to these in the former types of thrombolites, while in the inter-particle pore space, they were mostly full of anhydrite (Figure 3c, 4d). In the Tongshen-1 well, the anhydrite in the pores was partially dissolved, and residual anhydrite with an irregular edge could be found in the pores (Figure 4e).

Pore Types
Six types of pores were identified in T2l4 3 : inter-clot pores, fenestral pores, intra-clot pores, inter-crystal pores, micropores, and fractures. The terminology of pore types refers to the pore classification scheme of carbonates proposed by Moore [54].
The inter-clot pores in T2l4 3 were well developed in facies A and facies B, in which the particles were mainly interconnected microbial clots, and the pores had irregular shapes, with sizes ranging from tens to hundreds of microns (Figure 5a, e). Dolomite cements grew close to the clots and occupied part of pore space. The rest of the pore space was either filled with calcite cements or preserved without any fillings.
Fenestral pores were mainly developed in facies C. The pores were developed directionally along the laminations, with a size of 50 to 1500 μm (Figure 5b). The shape of the Another type of thrombolite is anhydritic thrombolites (facies B), which was developed mainly in T 2 l 4 1 and T 2 l 4 2 . The characteristics of the clots in the anhydritic thrombolites were similar to these in the former types of thrombolites, while in the inter-particle pore space, they were mostly full of anhydrite (Figures 3c and 4d). In the Tongshen-1 well, the anhydrite in the pores was partially dissolved, and residual anhydrite with an irregular edge could be found in the pores (Figure 4e).

Stromatolites
Stromatolites were mainly developed in T 2 l 4 3 and were easy to recognize from the cores because of their slightly undulated laminations ( Figure 3d). A large number of fenestral pores could be found in the core (Figure 3e). The alternation of dark and light dolomite (millimeters in thickness for a single lamination) showed that the stromatolites often developed in an environment where water energy changes frequently. There are two types of undulate laminations in stromatolites. One is made up of dolomicrite, and the other is composed of peloids and clots, while both can be found in one single lamination occasionally (Figure 4f).

Bioclastic Packstone
A large number of benthic foraminifera (dominated by miliolinid foraminifera) and a few fragments of shells could be found in bioclastic packstone. Besides, it contained a small number of peloids. The matrix was primarily dolomicrite, and little pore space occurred in bioclastic packstone.

Grainstone
Three types of grainstone were found in T 2 l 4 3 : intraclast-dominated grainstone (facies E), microbial clot-dominated grainstone (facies F) ( Figure 4g) and peloid-dominated packstone to grainstone (facies G) ( Figure 4h). However, in general, these three kinds of grainstones coexist in a sample. The intraclasts were composed of uniform dolomicrite and subrounded in sizes of 100 to 300 µm. The microbial clots were isolated and differed from the intraclasts by their irregular shape and internal agglutinated structure of microorganisms, and they were generally larger than intraclasts and peloids, with sizes ranging mainly between 200 to 1500 µm. The peloids were subrounded or rounded, had a good sorting, were composed of uniform dolomicrite, and commonly exhibited a diameter from 30 to 80 µm. Some inter-particle pores were found in grainstones. The pore sizes of intraclast-dominated grainstone and microbial clot-dominated grainstone were mainly tens of microns, while the pores in peloid-dominated grainstone had a size of hundreds of nanometers to several microns.
The anhydrite shows milky white in the cores, while dolomite shows dark grey and had a reticulated distribution in anhydrite ( Figure 3f). Observation under a microscope suggested that the anhydrite crystals were platy or had an irregular shape, and the reticulated dolomite turned out to be formed by microorganism activities (Figure 4i). Little pore space could be found in anhydrite and dolomitic anhydrite.

Crystalline Carbonates
Crystalline carbonates (facies J) developed mainly in the upper sub-member of Lei-4. The calcite was mainly less than 1 µm, and the dolomite displayed a wide range of crystal sizes, from 1 µm to 250 µm. The residual microbial texture could be found in silt crystal dolomite.

Pore Types
Six types of pores were identified in T 2 l 4 3 : inter-clot pores, fenestral pores, intra-clot pores, inter-crystal pores, micropores, and fractures. The terminology of pore types refers to the pore classification scheme of carbonates proposed by Moore [54]. The inter-clot pores in T 2 l 4 3 were well developed in facies A and facies B, in which the particles were mainly interconnected microbial clots, and the pores had irregular shapes, with sizes ranging from tens to hundreds of microns (Figure 5a,e). Dolomite cements grew close to the clots and occupied part of pore space. The rest of the pore space was either filled with calcite cements or preserved without any fillings.
fenestral pores was irregular and narrow. Abundant pore space was preserved, although few dolomite and quartz cements filled in the pore space. Intra-clot pores were observed in facies A and facies B, which were formed in microbial clots, with sizes ranging from 2 to 15 μm (Figure 5a). Inter-crystal pores were mainly developed in facies A and facies C ( Figure 5d). While intercrystal pores represented a smaller proportion of the total porosity than inter-particle pores (between interconnected microbial clots), this type of pore was widely developed in microbial carbonate rocks. Inter-crystal pores were 5 to 15 μm in size, and little cement was formed in the pore space.
Micropores were observed in facies A, facies C, and facies G, and they were clearly recognized under SEM observation. The types of micropores were mainly inter-crystal pores, inter-particle pores, and microbial pores which developed between filaments of microbes, with a size of 200 nm to 1μm (Figure 5d, f).

Physical Properties
The petrophysical data, including porosity and permeability, were obtained from 12 wells. On the whole, the microbial reservoirs of Lei-4 had the characteristics of a medium-low porosity and low permeability, based on statistics from 347 samples ( Figure 6). The physical properties varied with lithofacies. The microbial carbonates had a better reservoir quality than grainstone, bioclastic packstone, and anhydrite. In the microbial carbonates, the porosity of (f) Inter-particle pores (micropores, yellow) and peloids (red arrow), T 2 l 4 3 , Longshen-1 well, 5971.46 m.
Fenestral pores were mainly developed in facies C. The pores were developed directionally along the laminations, with a size of 50 to 1500 µm (Figure 5b). The shape of the fenestral pores was irregular and narrow. Abundant pore space was preserved, although few dolomite and quartz cements filled in the pore space.
Intra-clot pores were observed in facies A and facies B, which were formed in microbial clots, with sizes ranging from 2 to 15 µm (Figure 5a).
Both structural fractures and dissolution fractures were generally developed in facies A and facies F. The aperture of the structural fractures was mainly from 0.01 to 0.2 mm (Figure 5c). Dissolution fractures were mainly 0.1~0.3 mm in aperture and were enlarged by the dissolution in a partial position of the fractures.
Inter-crystal pores were mainly developed in facies A and facies C (Figure 5d). While inter-crystal pores represented a smaller proportion of the total porosity than inter-particle pores (between interconnected microbial clots), this type of pore was widely developed in microbial carbonate rocks. Inter-crystal pores were 5 to 15 µm in size, and little cement was formed in the pore space.
Micropores were observed in facies A, facies C, and facies G, and they were clearly recognized under SEM observation. The types of micropores were mainly inter-crystal pores, inter-particle pores, and microbial pores which developed between filaments of microbes, with a size of 200 nm to 1 µm (Figure 5d,f).

Physical Properties
The petrophysical data, including porosity and permeability, were obtained from 12 wells. On the whole, the microbial reservoirs of Lei-4 had the characteristics of a medium-low porosity and low permeability, based on statistics from 347 samples ( Figure 6). The physical properties varied with lithofacies. The microbial carbonates had a better reservoir quality than grainstone, bioclastic packstone, and anhydrite. In the microbial carbonates, the porosity of thrombolites ranged from 0.26 to 23.7%, with an average of 3.77%. The average porosity of limestone was 0.76%, while average porosity of dolomite was 4.8%. This shows that dolomite had a better reserve capacity than limestone in Lei-4. While the stromatolites had a high porosity of 7.02%. thrombolites ranged from 0.26 to 23.7%, with an average of 3.77%. The average porosity of limestone was 0.76%, while average porosity of dolomite was 4.8%. This shows that dolomite had a better reserve capacity than limestone in Lei-4. While the stromatolites had a high porosity of 7.02%. The analysis of main pore types in thrombolites was carried out using the software, Image-J ( Figure 7). The main pore types of thrombolites were inter-clot pores, intra-clot pores and inter-crystal pores. According to the statistics results (Table 2), the inter-crystal pores had an average area of 0.12 × 10 6 μm 2 , accounting for 8.71% of the total pores. The average area of the intra-clot pores was 0.07 × 10 6 μm 2 , accounting for 6.40% of the total pores, while the average area of the inter-clot pores was 1.23 × 10 6 μm 2 , accounting for 84.89% of the total pores, which indicates that inter-clot pores were the dominant pore type of the microbial reservoir.  The analysis of main pore types in thrombolites was carried out using the software, Image-J ( Figure 7). The main pore types of thrombolites were inter-clot pores, intra-clot pores and inter-crystal pores. According to the statistics results (Table 2), the inter-crystal pores had an average area of 0.12 × 10 6 µm 2 , accounting for 8.71% of the total pores. The average area of the intra-clot pores was 0.07 × 10 6 µm 2 , accounting for 6.40% of the total pores, while the average area of the inter-clot pores was 1.23 × 10 6 µm 2 , accounting for 84.89% of the total pores, which indicates that inter-clot pores were the dominant pore type of the microbial reservoir.
thrombolites ranged from 0.26 to 23.7%, with an average of 3.77%. The average porosity of limestone was 0.76%, while average porosity of dolomite was 4.8%. This shows that dolomite had a better reserve capacity than limestone in Lei-4. While the stromatolites had a high porosity of 7.02%. The analysis of main pore types in thrombolites was carried out using the software, Image-J ( Figure 7). The main pore types of thrombolites were inter-clot pores, intra-clot pores and inter-crystal pores. According to the statistics results (Table 2), the inter-crystal pores had an average area of 0.12 × 10 6 μm 2 , accounting for 8.71% of the total pores. The average area of the intra-clot pores was 0.07 × 10 6 μm 2 , accounting for 6.40% of the total pores, while the average area of the inter-clot pores was 1.23 × 10 6 μm 2 , accounting for 84.89% of the total pores, which indicates that inter-clot pores were the dominant pore type of the microbial reservoir.

Redox-Sensitive Trace Elements
The values of Th, U, V, Ni, Mn, Sr, and Co of the microbial carbonates (thrombolites and stromatolites) and grainstone are shown in Table 3 The diagenetic alteration may have affected the representativeness of the carbonate samples of seawater, and the diagenetic alteration process of marine carbonate was a process of manganese acquisition and strontium loss [55][56][57]. Therefore, the concentrations of Mn and Sr and the ratio of Mn/Sr were used to determine the degree of diagenetic alteration. In general, carbonates with Mn/Sr < 10 retained their original seawater geochemical conditions, especially for Mn/Sr < 2-3 [58,59]. In this study, the average Mn/Sr ratio of all the samples was 0.114 (far less than 2), which indicated that their geochemical data could represent the original redox conditions of depositional environments.

Rare Earth Elements
The values of rare earth elements (REE) are presented in Table 4 and Figure 8. The REE of microbial carbonates (thrombolites and stromatolites) and grainstone varied from 0.50 to 3.32 (average: 1.20), which is two magnitudes lower than the average REE of PAAS (184.77 µg/g). This indicates that the sediments in T 2 l 4 were weakly affected by terrigenous clasts during the sedimentary process or diagenesis [52]. The LREE (light rare earth elements)/ HREE (heavy rare earth elements) varied from 0.70 to 8.07 (average: 1.87), which indicated that the LREE were lightly enriched, and HREE were lightly depleted. Almost all samples of the Leikoupo Formation displayed a positive Eu anomaly. The samples of lithofacies G and J displayed a slightly positive Ce anomaly, while the microbialites samples displayed a slightly negative Ce anomaly.

Stable Carbon and Oxygen Isotopes
The results of stable C and O isotopes are presented in Table 5 and Figure 9. The δ 18 O VPDB value of limestone was between −8.43% and −7.02% , the δ 13 C VPDB values were between 1.09% and 2.25% , and the δ 18 O VPDB value of dolomite ranged from −9.97% to −3.50% and from 2.16% to 3.75% for the δ 13 C VPDB values. The δ 18 O VPDB value of thrombolites was between −9.55% and −3.50% and between 1.89% and 3.75% for the δ 13 C VPDB values. The δ 18 O VPDB value of stromatolites ranged from −9.97% to −6.02% and from 2.32% to 2.69% for the δ 13 C VPDB values. The δ 18 O VPDB value of grainstone was between −7.95% and −5.77% , and the δ 13 C VPDB values were between 1.09% and 2.77% . In general, thrombolites displayed the highest isotopic range for both the δ 18 O VPDB and

Stable Carbon and Oxygen Isotopes
The results of stable C and O isotopes are presented in Table 5 and Figure 9. The δ 18 OVPDB value of limestone was between -8.43‰ and -7.02‰, the δ 13 CVPDB values were between 1.09‰ and 2.25‰, and the δ 18 OVPDB value of dolomite ranged from -9.97‰ to -3.50‰ and from 2.16‰ to 3.75‰ for the δ 13 CVPDB values. The δ 18 OVPDB value of thrombolites was between -9.55‰ and -3.50‰ and between 1.89‰ and 3.75‰ for the δ 13 CVPDB values. The δ 18 OVPDB value of stromatolites ranged from -9.97‰ to -6.02‰ and from 2.32‰ to 2.69‰ for the δ 13 CVPDB values. The δ 18 OVPDB value of grainstone was between -7.95‰ and -5.77‰, and the δ 13 CVPDB values were between 1.09‰ and 2.77‰. In general, thrombolites displayed the highest isotopic range for both the δ 18 OVPDB and δ 13 CVPDB values, grainstone displayed the lowest range for the δ 18 OVPDB values, and stromatolites displayed the lowest range for the δ 13 CVPDB values.

Stable Carbon and Oxygen Isotopes
The results of stable C and O isotopes are presented in Table 5 and Figure 9. The δ 18 OVPDB value of limestone was between -8.43‰ and -7.02‰, the δ 13 CVPDB values were between 1.09‰ and 2.25‰, and the δ 18 OVPDB value of dolomite ranged from -9.97‰ to -3.50‰ and from 2.16‰ to 3.75‰ for the δ 13 CVPDB values. The δ 18 OVPDB value of thrombolites was between -9.55‰ and -3.50‰ and between 1.89‰ and 3.75‰ for the δ 13 CVPDB values. The δ 18 OVPDB value of stromatolites ranged from -9.97‰ to -6.02‰ and from 2.32‰ to 2.69‰ for the δ 13 CVPDB values. The δ 18 OVPDB value of grainstone was between -7.95‰ and -5.77‰, and the δ 13 CVPDB values were between 1.09‰ and 2.77‰. In general, thrombolites displayed the highest isotopic range for both the δ 18 OVPDB and δ 13 CVPDB values, grainstone displayed the lowest range for the δ 18 OVPDB values, and stromatolites displayed the lowest range for the δ 13 CVPDB values.

Compaction
The mechanical compaction and pressure dissolution (i.e., chemical compaction) were largely developed in T 2 l 4 , whether in grainstone or microbial carbonates. In grainstone, many intraclasts had a ductile elliptical shape, and a concave-convex contact between particles could be observed. A large number of stylolites, filled with residual asphalt and little pyrite, could be observed (Figure 10a). Obviously, compaction greatly reduced the initial porosity.
crystals precipitated (Figure 10g-h). During the deep burial period, the residual pores was completely filled with clear coarse calcite spar (Figure 10i).

Dolomitization
The dolomite in Lei-4 experienced relatively early dolomitization. The dolomite had an aphanocrystalline to fine crystal size and a dirty surface. The δ 13 CVPDB values of limestone ranged from 1.09‰ to 2.25‰ (average 1.73‰), and the δ 13 CVPDB values of dolomite ranged from 2.16‰ to 3.75‰ (average 2.56‰), which shows a small difference, so the fluid for dolomitization probably originated from the seawater. In addition, dolomitization preceded the formation of gypsum and anhydrite, which precipitated in the inter-clot pores and fenestral pores. Besides, although it underwent the burial period, it seems that the dolomite was not affected by the relatively strong diagenetic alteration through the analysis of the geochemical data of the whole rock. It lacked the invasion of external fluids, and Machel [30] and Jiang [67] believe that it belongs to the early diagenesis. The evidence presented above indicates that main dolomitization occurred in the relatively early diagenesis stage.

Dissolution
In the study area, Lei-4 was mainly subjected to two stages of dissolution. The first stage was the dissolution in the syn-depositional period, which was formed by high-frequency sea-level changes and had the characteristics of fabric selectivity. It was mainly developed in microbial carbonates, which was due to the high terrain of microbial mounds. During the short-term sea-level fall period, the mounds were exposed to meteoric freshwater to undergo leaching, forming a large number of dissolution pores (Figure 4e). The second stage of dissolution was the dissolution during the burial period, which was weaker than that of syn-depositional dissolution and was characterized by expansion dissolution along the crack or on the basis of early dissolution pores (Figure 10b). The reason for the smaller scale of burial dissolution was that the fluid velocity in the pores was abnormally slow in the deep burial environment. If there is no fracture or hydrothermal influence, the formation water and the surrounding rock are in equilibrium, and the carbonate-formation water reaction was a balancing process under a nearly closed system [63]. In the deep burial system, the equilibrium of the dissolution-precipitation reaction tended in the direction of precipitation as the burial depth increased, which is embodied by a small amount of precipitation of carbonate minerals, while the short-term opening environment (the development of cracks or the injection of acidic fluids) led to a small amount of dissolution of carbonate minerals [64]. Besides, through thermodynamic simulation calculation, after oil and gas charging, the proportion of water-rock in the carbonate reservoir was low, and increasing the porosity of 1% of limestone, with a thickness of 100 m, required 37,000 cubic meters of pore water, while the closed systems did not have sufficient pore water [65,66]. Thus, deep dissolution played an important role in adjusting the early pores in T 2 l 4 , but its contribution to increasing the reservoir was not significant.

Cementation
Lei-4 experienced multi-stage cementation, which caused reduction of the reservoir quality. After the formation of a microbial framework structure by bonding marl, due to the arid climate and sea level fall, some saline minerals, such as gypsum, precipitated and completely or partially filled between the clots (Figure 4d). Thanks to the syn-depositional dissolution, the gypsum dissolved and pores formed, starting with the precipitate of fibrous high-magnesium calcite around grains. Due to diagenetic alteration, the calcite was dog-tooth, isopachous, and circumgranular ( Figure 10c) and was replaced by dolomites later on. Cathodoluminescence analysis showed that the dolomite cement had a dark red light, which was similar to the matrix (Figure 10d,e). This implies that the pore water was probably derived from coeval seawater. Combined with its unique morphology, it indicates that the dolomite cementation occurred in a marine-phreatic environment. As the burial depth increased, the euhedral dolomite crystals slowly precipitated in the pores (Figure 10f). During the medium-deep burial period, the circulation of fluids in the pores was limited. After the precipitate of second-stage dolomite crystals, the third phase of larger dolomite and a small number of quartz crystals precipitated (Figure 10g,h). During the deep burial period, the residual pores was completely filled with clear coarse calcite spar (Figure 10i).

Dolomitization
The dolomite in Lei-4 experienced relatively early dolomitization. The dolomite had an aphanocrystalline to fine crystal size and a dirty surface. The δ 13 C VPDB values of limestone ranged from 1.09% to 2.25% (average 1.73% ), and the δ 13 C VPDB values of dolomite ranged from 2.16% to 3.75% (average 2.56% ), which shows a small difference, so the fluid for dolomitization probably originated from the seawater. In addition, dolomitization preceded the formation of gypsum and anhydrite, which precipitated in the inter-clot pores and fenestral pores. Besides, although it underwent the burial period, it seems that the dolomite was not affected by the relatively strong diagenetic alteration through the analysis of the geochemical data of the whole rock. It lacked the invasion of external fluids, and Machel [30] and Jiang [67] believe that it belongs to the early diagenesis. The evidence presented above indicates that main dolomitization occurred in the relatively early diagenesis stage.

Interpretation of Geochemical Characteristics
In general, the LREE of oxidized water was more likely to enter the Mn-Fe oxide or hydroxide than HREE, so the rare earth elements in the oxidized water generally showed a loss of LREE [68,69]. Reductive conditions led to the dissolution of oxides or hydroxide, resulting in the presence of LREE-rich features in weak oxygen and anoxic waters [68,69]. Accordingly, the sedimentary seawaters of Lei-4 were anoxic. Besides, the REE pattern of different lithofacies in the Leikoupo Formation showed a relatively flat distribution pattern (Figure 9). The reason for this distribution pattern was mainly the participation of microorganisms. In a microbial environment, the carbonate nucleus was more easily formed, and the shape of the crystal was mainly dumbbell-shaped, which is different from chemically precipitated carbonate. These morphological features had an important influence on the entry of elements into the crystal [70], thus affecting the enrichment of rare earth elements. Carbonate rocks (partially microbial stromatolites) of the Precambrian and North China Craton in Eastern China also had a relatively flat distribution pattern of REE [71].
The values of the Ce anomaly were used to determine redox conditions [52,72]. In an oxic environment, Ce 3+ was oxidized to Ce 4+ so that the concentration of Ce 3+ decreased, the negative anomaly of Ce was formed, and the value of the Ce anomaly was less than 1. On the other hand, in an anoxic environment, the concentration of Ce 3+ increased, the positive anomaly of Ce was formed, and the value of the Ce anomaly was larger than 1 [52,72]. In T 2 l 4 , MREE is enriched (Figure 8), and the lithofacies G and J sample displayed a slight positive Ce anomaly (varied from 1.010 to 1.052, with an average of 1.024) ( Table 4 and Figure 8), which is compelling evidence indicating an anoxic marine environment [73][74][75]. However, a slight negative Ce anomaly is observed in the microbialites (Facies A, B, and C) samples varied from 0.877 to 1.084 (average 0.962), which indicates an oxic environment of microbialites. The analysis of trace elements makes it clear that the sea water was anoxic. Therefore, we argue that the slightly negative Ce of microbialites is probably due to that the microbialites were exposed to subaerial conditions in the eogenetic period.
In a low-temperature and alkaline environment, Eu 3+ was reduced to relatively soluble Eu 2+ , the concentration of Eu 3+ was decreased, the negative anomaly of Eu occurred, and the value of the Eu anomaly was less than 1. Conversely, in a high-temperature environment, the positive anomaly of Eu was formed, the value of the Eu anomaly was larger than 1, and the significant Eu positive anomaly was mainly affected by hydrothermal fluids [72,[76][77][78]. In T 2 l 4 , the Eu anomaly varied from 0.559 to 63.876 (average: 8.576, which is much larger than 1) ( Table 4 and Figure 8), which indicates that the carbonates in Lei-4 were influenced by hydrothermal fluids in diagenesis, and a small amount of hydrothermal fluorite and sphalerite were also found in the cores [52].
In view of the global Triassic seawater carbon isotope variation curve [79], the biological extinction in the Late Permian led to a reduction of the carbon isotope values (−2% ). In the Triassic, the carbon isotope values began to elevate due to biological recovery during the Annie period (sedimentary period of the Leikoupo Formation), and the carbon isotope value ranged from −1% to 2% . The carbon isotope values of the microbial carbonates in the Leikoupo Formation ranged from 1.09% to 3.75% (Figure 10), which indicates a positive shifting, compared to the seawater in the same period ( Figure 10). There was no coal deposition on the land, so the amount of primitive organic carbon created by terrestrial plants was limited in this period [80]. However, the fixation of CO 2 and the converse of organic carbon to the CO 2 of microbes (e.g., bacteria and algae) could produce a measurable value of δ 13 C rising [81]. This has also been proven in the modern marine carbon cycle [82]. Microbes were the first organisms to recover after the Great Extinction, and their photosynthesis reconstructed the isotope fractionation effect of organic carbon destroyed by extinction [80]. Therefore, it is possible that the reason for the positive shift was the large number of microbes in this period.
In the Middle Triassic, the global oxygen isotope values of marine carbonates ranged from −6% to −1% [79], while the oxygen isotope values of the microbial carbonates in the Leikoupo Formation ranged from −9.97% to −3.5% (Figure 10), with a negative shift feature. This indicates that the microbial carbonate rocks experienced a meteoric freshwater diagenetic environment in the early stages [83].

Lagoon
Due to the arid climate, the seawater in Anisian was restricted and envaporitic, and the dolomitic lagoon and gypsiferous lagoon stably developed ( Figure 11). The dolomitic lagoon was mainly developed in T 2 l 4 3 and was about 10 to 38 m thick. In the study area, the dolomitic lagoon was dominated by dark gray and microcrystalline to very fine crystalline, and its lithofacies included thrombolites (facies A), bioclastic packstone (facies D) and peloid-dominated packstone to grainstone (facies G). The seawater circulation of the dolomitic lagoon was relatively blocked, so the water energy was weak and miliolinid foraminifera, and few fragments of shells could be found in it. The gypsiferous lagoon developed in T 2 l 4 1 and T 2 l 4 2 and was more than 200 m thick. The lithofacies in the gypsiferous lagoon contained anhydrite (facies H), dolomitic anhydrite (facies I), and a small amount of anhydritic thrombolites (facies B), which were dominated by a light white to off-white color and were fine to coarse crystalline ( Figure 12).

Microbial Mound
Microbial mounds were mainly developed in the intertidal zone in T 2 l 4 3 ( Figure 11). They were the most widely developed, were about 20 to 48 m thick, and were chiefly composed of thrombolites (facies A) and stromatolites (Facies C) and a small amount of peloid-dominated packstone to grainstone (facies G). The seawater circulation and the hydrodynamic condition were relatively strong, so an undulated laminated microbial structure was dominant. Miliolinid foraminifera and tectulariid foraminifera (single chamber and biserial chamber) were found in the microbial mounds ( Figure 12).

Shoal
Shoals were developed in the intertidal zone, with a thickness of about 15 to 38 m ( Figure 11). The thickest part was located near the Pengzhou 103 well and had a thickness of 57 m. The lithology combination was dominated by intraclast-dominated grainstone (facies E), microbial clot-dominated grainstone (facies F) and peloid-dominated packstone to grainstone (facies G). Most of the grains were cemented by bright granulous dolomite or calcite, indicating that the hydrodynamic condition was strong, and a small amount of miliolinid foraminifera debris was preserved. The subtidal zone was subdivided into a shallow subtidal subzone and deep subtidal subzone in the study area. The shallow subtidal subzone was dominated by lagoons, while the deep subtidal subzone was less developed in the western margin of the study area, with argillaceous carbonates, dolomicrite, and micrite developed. The deep subtidal subzone was located below the wave base, so the hydrodynamic condition was weak ( Figure 12). Figure 11. Mineralogy, textures, facies, porosity, and permeability of the middle and upper sub-member of the fourth member of the Leikoupo Formation. Figure 11. Mineralogy, textures, facies, porosity, and permeability of the middle and upper sub-member of the fourth member of the Leikoupo Formation.
Energies 2020, 13, x; doi: FOR PEER REVIEW www.mdpi.com/journal/energies condition was strong, and a small amount of miliolinid foraminifera debris was preserved. The subtidal zone was subdivided into a shallow subtidal subzone and deep subtidal subzone in the study area. The shallow subtidal subzone was dominated by lagoons, while the deep subtidal subzone was less developed in the western margin of the study area, with argillaceous carbonates, dolomicrite, and micrite developed. The deep subtidal subzone was located below the wave base, so the hydrodynamic condition was weak ( Figure 12).

Eogenetic (Near-Surface) Period
During the eogenetic period, the original calcite marls were captured and bound by microorganisms, the microbial fenestral pores and inter-particle pores were formed, and the seawater was filled in the pores. In the syn-depositional period, extensive dolomitization occurred, and the degree of crystallinity of dolomite crystals was low. The order degree of dolomite was between 0.38 and 0.89 (average 0.64), which was generally low. The early dolomitization could add some pore space due to volume shrinkage, which enhanced the anti-compaction ability of the rocks. Subsequently, the early seawater gradually evaporated and concentrated due to the arid climate, as discussed before, and gypsum gradually precipitated in the microbial fenestral and inter-particle pores. Due to the relative sea-level fall, the gypsum was exposed to the surface and was completely or partially leached by meteoric water. Residual gypsum with an irregular edge could be found in pores. During the relative sea-level rise, the fluid in the pores was converted into seawater, and the fibrous high-magnesium calcite was cemented and subsequently replaced by dolomite. The crystal size of dolomite was very fine to fine. In addition to microbial fenestral pores, intra-particle pores were also developed in this period ( Figure 13). discussed before, and gypsum gradually precipitated in the microbial fenestral and inter-particle pores. Due to the relative sea-level fall, the gypsum was exposed to the surface and was completely or partially leached by meteoric water. Residual gypsum with an irregular edge could be found in pores. During the relative sea-level rise, the fluid in the pores was converted into seawater, and the fibrous high-magnesium calcite was cemented and subsequently replaced by dolomite. The crystal size of dolomite was very fine to fine. In addition to microbial fenestral pores, intra-particle pores were also developed in this period ( Figure 13). Figure 13. Evolution of microbial inter-particle pores in the Leikoupo Formation. (a) In the sedimentary period, initial inter-particle pores formed, and gypsum precipitated from seawater; (b) In the syn-depositional period, gypsum and a few clots dissolved, and inter-particle pores and intraparticle pores formed; (c) In shallow burial period, circumgranular and dog-tooth cements formed in the pores; (d) As the depth increased, euherdral dolomite precipitated; (e) Quartz crystal precipitated in the deep burial period; (f) Clear coarse calcite spar precipitated in the deep burial period.

Mesogenetic (Burial) Period
During the mesogenetic period, the temperature and pressure increased as the burial depth was increased, and the residual gypsum was dehydrated and converted into anhydrite. In addition, a large number of stylolites were formed. Some of the particles were dissolved, and oil and gas charged during this period. Residual bitumen could be found in the stylolites and around some of the secondstage dolomite crystals. The circulation of the fluid in the pores was limited, and after the secondstage dolomite crystals, the third phase of the larger dolomite and a small amount of single-crystal quartz crystals precipitated. The residual reservoir space in some pores was completely filled by clear coarse calcite during this period, causing damage to the reservoir porosity. Due to the release of stress during the burial process and the influence of structural uplift, three-stage fractures occurred. Acidic or other unsaturated fluids entered the fractures, leading to the occurrence of dissolution, which had a certain positive impact on the reservoir, but the impact was limited due to its low occurrence frequency ( Figure 14). Figure 13. Evolution of microbial inter-particle pores in the Leikoupo Formation. (a) In the sedimentary period, initial inter-particle pores formed, and gypsum precipitated from seawater; (b) In the syn-depositional period, gypsum and a few clots dissolved, and inter-particle pores and intra-particle pores formed; (c) In shallow burial period, circumgranular and dog-tooth cements formed in the pores; (d) As the depth increased, euherdral dolomite precipitated; (e) Quartz crystal precipitated in the deep burial period; (f) Clear coarse calcite spar precipitated in the deep burial period.

Mesogenetic (Burial) Period
During the mesogenetic period, the temperature and pressure increased as the burial depth was increased, and the residual gypsum was dehydrated and converted into anhydrite. In addition, a large number of stylolites were formed. Some of the particles were dissolved, and oil and gas charged during this period. Residual bitumen could be found in the stylolites and around some of the second-stage dolomite crystals. The circulation of the fluid in the pores was limited, and after the second-stage dolomite crystals, the third phase of the larger dolomite and a small amount of single-crystal quartz crystals precipitated. The residual reservoir space in some pores was completely filled by clear coarse calcite during this period, causing damage to the reservoir porosity. Due to the release of stress during the burial process and the influence of structural uplift, three-stage fractures occurred. Acidic or other unsaturated fluids entered the fractures, leading to the occurrence of dissolution, which had a certain positive impact on the reservoir, but the impact was limited due to its low occurrence frequency ( Figure 14).

Impact of Sedimentation and Diagenesis on the Reservoirs of Lei-4
From the tectonic evolution of the western margin of the Yangtze Block, the Middle Triassic belongs to the end stage of the development of the carbonate platform, and the subduction of the western margin of the Yangtze Block to the Songpan Block did not respond to sedimentation. Due to the impact of the subduction, the paleo Tethys Ocean was gradually closed, and the seawater gradually withdrew from the Western Sichuan Basin along the NE-SW direction. The study area was located in the arid climatic zone, and the evolution of life was in the recovery period after the mass extinction. The microorganisms were prosperous in this period.

Impact of Sedimentation and Diagenesis on the Reservoirs of Lei-4
From the tectonic evolution of the western margin of the Yangtze Block, the Middle Triassic belongs to the end stage of the development of the carbonate platform, and the subduction of the western margin of the Yangtze Block to the Songpan Block did not respond to sedimentation. Due to the impact of the subduction, the paleo Tethys Ocean was gradually closed, and the seawater gradually withdrew from the Western Sichuan Basin along the NE-SW direction. The study area was located in the arid climatic zone, and the evolution of life was in the recovery period after the mass extinction. The microorganisms were prosperous in this period.
The prosperity of the microorganisms led to the development of microbial carbonate rocks in the Leikoupo Formation, and the microbial mounds were widely distributed in the study area, providing primary pores and a material basis for diagenetic alteration for the development of secondary pores. The behaviors of the microorganisms in capturing marls lacked regularity, resulting in an irregular shape of the clots in the thrombolites. In addition to providing a certain primary pore space, the irregular-shaped clots provided a relatively early anti-compaction skeleton of the primary pores after its consolidation during diagenesis. The microbial mounds had a relatively positive relief, which was conducive to syn-depositional dissolution, laying a foundation for the development of high-quality reservoirs. In addition, from the lateral correlation of the Lei-4 reservoirs, most of the high-quality reservoirs were also developed in the microbial mounds ( Figure 11).
The diagenesis affecting the development of microbial carbonate reservoirs in the Leikoupo Formation was mainly early dolomitization and syn-depositional dissolution. Early dolomitization allowed early pores to be effectively preserved. Firstly, in the case of constant porosity, dolomite could create a pore structure (pore size and connectivity) that is better than calcite. Besides, early The prosperity of the microorganisms led to the development of microbial carbonate rocks in the Leikoupo Formation, and the microbial mounds were widely distributed in the study area, providing primary pores and a material basis for diagenetic alteration for the development of secondary pores. The behaviors of the microorganisms in capturing marls lacked regularity, resulting in an irregular shape of the clots in the thrombolites. In addition to providing a certain primary pore space, the irregular-shaped clots provided a relatively early anti-compaction skeleton of the primary pores after its consolidation during diagenesis. The microbial mounds had a relatively positive relief, which was conducive to syn-depositional dissolution, laying a foundation for the development of high-quality reservoirs. In addition, from the lateral correlation of the Lei-4 reservoirs, most of the high-quality reservoirs were also developed in the microbial mounds ( Figure 11).
The diagenesis affecting the development of microbial carbonate reservoirs in the Leikoupo Formation was mainly early dolomitization and syn-depositional dissolution. Early dolomitization allowed early pores to be effectively preserved. Firstly, in the case of constant porosity, dolomite could create a pore structure (pore size and connectivity) that is better than calcite. Besides, early dolomitization could enhance the rock anti-compaction ability. Early dolomitization could also inhibit compaction during the burial period and was conducive to the preservation of reservoir space. In addition, the syn-depositional dissolution of the gypsum greatly increased the reservoir space.

Preservation of Pores in Microbial Carbonate Reservoirs
In addition to the preservation of pores by early dolomitization, as mentioned above, the hydrocarbon charging and rapid burial also preserved early pores. Residual bitumen was visible around the second stage of dolomite crystals, which inhibited the further cementation during the burial process (Figure 9f). In addition, from the burial history of the CK-1, GK-1, and PG-1 wells, the Longmen Mountains were rapidly uplifted in the Late Triassic [16,84]. The Xujiahe Formation, the overlying strata of the Leikoupo Formation, deposited great mudstones. The rapid burial caused the Leikoupo Formation to terminate the early near-surface cementation and to create a closed system. The closed system was also conducive to the preservation of early pores. In addition, the thick mudstone in the Xujiahe Formation played a good role in the protection of reservoirs as a high-quality seal.
Micropores is a noteworthy pore type in the Leikoupo Formation for they can contribute to total pore volume, increasing storage capacity for hydrocarbons. The micropores in carbonates with high organic matters, similar to the Leikoupo Formation, originate from the dissolution of organic acid or degradation with the proceeding of the thermal maturation of organic matter in some cases [85,86]. Besides, microbial activity could contribute to the formation of micropores. Micropores that result from the decay of eubacteria are common in travertines [87]. Bosak, Souza-egipsy, Corsetti and Newman [88] found the micropores formed in bacterially-induced calcite. Carbonates in the Leikoupo Formation are typical microbialites which provide the condition for development of micropores. No matter how the micropores formed, the hydrocarbon occluded the micropores could preserved those pores from destructive diagenetic alteration. The occurrence of oil or residual bitumen could indicate the hydrocarbon source and preservation mechanism of micropores [89]. If oil appear as inclusion, this indicates entrapment of oil inclusions after hydrocarbon migration, so the micropores preserved by early oil migration and charging. If oil and bitumen occupy intracrystal pores do not have features of inclusions, which indicates their in situ source, the micropores preserved by maturation of organic matter. However, the data of oil droplets or asphalt within micropores are needed to further explore the preservation of micropores.

Conclusions
The Leikoupo Formation in the Western Sichuan Basin of SW China developed microbial carbonate reservoirs. The reservoirs mainly occurred in T 2 l 4 3 , with the main rock types of thrombolites and stromatolites. The pore types were dominated by microbial inter-clot pores, intra-clot pores, and fenestral pores, formed by trapping and binding of marls by benthic microbial communities. Geochemical analysis shows that the redox condition of the sea water in the period of Lei-4 deposition was anoxic. Both the flat distribution pattern of rare earth elements and the positive drift of carbon isotopes were closely related to microorganisms. The negative drift of oxygen isotope was related to meteoric fresh water, which promoted the formation of early secondary pores.
Microbial carbonate reservoirs in Lei-4 were controlled by sedimentation and diagenesis. Thrombolites and stromatolites are the main high-quality reservoirs rock types developed as microbial mounds. The primary pores, developed in thrombolites and stromatolites, offered the space for diagenetic transformation. The precipitation of gypsum that occurred in synchronous or syn-depositional period was the material basis for the formation of secondary pores. Early dolomitization and syn-depositional dissolution were crucial to the development of the reservoirs.
The four stages of cementation during the burial period destroyed the reservoir to some extent, but the early dolomitization enhanced the anti-compaction ability of microbial carbonates during burial stage. Besides, the hydrocarbon filling after the second stage of cementation inhibited the intrusion of saturated fluid during the subsequent burial period. In addition, rapid burial accelerated the transformation of the reservoirs into a closed system and reduced early cementation.